Bipolar cell networks underlying steady-state intensity encoding in intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) encode ambient light intensity at steady-state and drive physiology even in the absence of melanopsin, but the synaptic basis of such encoding remains unclear. Using ultrastructural reconstructions, we mapped specific bipolar cell (BC) types and synapses conveying photoreceptor input to ipRGCs. Functional imaging showed BC glutamate release onto ipRGCs encodes intensity at steady-state, though release onto other RGCs also exhibits such encoding. Disrupting inhibition on BCs spared intensity-encoding release at ipRGC strata but reduced it elsewhere, consistent with inhibition shifting BC dynamic range. Recording postsynaptic excitatory currents showed that ipRGCs better preserve BC-derived intensity encoding than conventional RGCs. Thus, ipRGCs receive excitation from selected, inhibition-resistant BCs whose steady-state release encodes intensity. This, together with the enhanced preservation of postsynaptic intensity encoding, ensures reliable ipRGC intensity signaling independent of visual contrast to drive physiology and behavior.